Electromagnetic radiation emitting semiconductor chip and procedure for its production

ABSTRACT

A semiconductor chip which emits electromagnetic radiation is presented. The chip includes an epitaxially produced semiconductor layer stack based on nitride semiconductor material, which includes an n-conducting semiconductor layer, a p-conducting semiconductor layer, and an electromagnetic radiation generating region, which is arranged between these two semiconductor layers. The chip further includes a base on which the semiconductor layer stack is arranged, and a mirror layer, which is arranged between the semiconductor layer stack and the base. The n-conducting semiconductor layer faces away from the base, and the n-conducting semiconductor layer or an outcoupling layer located on the n-conducting semiconductor layer has a radiation-outcoupling surface which, in turn, includes planar outcoupling sub-surfaces, which are positioned obliquely with respect to a main plane of the radiation-generating region and each form an angle of between 15° and 70° with this plane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. patent application Ser. No. 10/671,854, filed Sep. 25, 2003now U.S. Pat. No. 7,129,528. Priority is claimed on German Application No. 102 45 628.3, filed on Sep. 30, 2002. The entire content of application Ser. No. 10/671,854 is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a semiconductor chip which emits electromagnetic radiation, having an epitaxially produced semiconductor layer stack based on nitride semiconductor material, which includes an n-conducting semiconductor layer, a p-conducting semiconductor layer and an electromagnetic radiation generating region which is arranged between these two semiconductor layers, a base on which the semiconductor layer stack is arranged, and a mirror layer, which is arranged between the semiconductor layer stack and the base, and a method for fabricating a plurality of these semiconductor chips which emit electromagnetic radiation.

2. Description of the Related Art

The term semiconductor layer stack based on nitride III-V compound semiconductor material is intended to encompass all semiconductor layer stacks whose main properties are determined by a nitride III-V compound semiconductor material. Nitride III-V compound semiconductor materials are all semiconductor materials which have nitrogen at the V lattice site, in particular GaN, InGaN, AlGaN and InGaAln. The semiconductor layer stack may, for example, have a conventional pn junction, a double heterostructure, a single quantum well structure (SQW structure) or a multiple quantum well structure (MQW structure). Structures of this type are known to the person skilled in the art and are therefore not explained in more detail at this point.

Semiconductor chips of the type described above convert electrical energy into electromagnetic radiation or vice versa. To do this, they usually have a semiconductor diode structure, which generates electromagnetic radiation in what is known as an active area between a p-conducting semiconductor layer and an n-conducting semiconductor layer. One problem of chips of this type is the outcoupling of the maximum possible proportion of the radiation which is generated in the electromagnetic radiation generating region.

In the case of what is known as a thin-film LED chip (LED=light emitting diode), which generally comprises a radiation-generating semiconductor layer stack arranged on a base, the thin semiconductor layers are grown epitaxially on a growth substrate wafer, which is detached after the semiconductor layers have, for example, been rebounded onto a base.

A semiconductor chip of this type is known, for example, from DE 100 20 464 A1. In this case, a reflector is formed on a surface of a semiconductor layer stack which is located on the side of the semiconductor layer stack opposite to the surface from which light emerges out of the semiconductor layer stack. The reflector is formed by a dielectric mirror or by a reflective metallic contact surface, which at the same time is also used for establishing the electrical contact of the semiconductor body. To improve the outcoupling of light, the entire free surface of the semiconductor body is roughened in order to prevent total reflection at the outcoupling surface between the semiconductor body and the environment and to thereby increase the light yield of the semiconductor chip.

A drawback of this type of semiconductor chip is that the roughening of the outcoupling surface requires at least one additional method step, entailing additional time expenditure and therefore additional costs. Furthermore, the light yield of these semiconductor chips is still well short of the theoretical maximum.

Hitherto, the prior art has not disclosed any measures for improving the light yield of semiconductor chips of the type described above which are directed at the p-conducting semiconductor layer. A particular problem of p-conducting nitride III-V compound semiconductor layers is their sensitivity to chemical or mechanical treatment. Contacts with low contact resistances can only be produced with difficulty on surfaces which have been treated in this way. Therefore, surface patterning of the p-conducting semiconductor layer by etching or mechanical roughening is not advantageous.

Therefore, as an alternative the n-conducting semiconductor layer is generally textured by means of RIE (reactive ion etching). However, this method requires the n-conducting semiconductor layer to be detached from the growth substrate wafer. The side from which the growth substrate wafer has been removed must also first of all be planarized, so that the mask layers which are required for patterning can be applied. Further process steps are required in order to pattern the surface in a suitable etching installation and then to remove the mask layer.

SUMMARY OF THE INVENTION

An object of the invention is to provide a semiconductor chip of the type described above with an increased light yield and without great technical effort.

In one aspect, the invention relates to a semiconductor chip that emits electromagnetic radiation. The semiconductor chip includes an epitaxially produced semiconductor layer stack based on nitride semiconductor material, which includes an n-conducting semiconductor layer, a p-conducting semiconductor layer, and an electromagnetic radiation generating region, which is arranged between these two semiconductor layers. The semiconductor chip further includes a base on which the semiconductor layer stack is arranged, and a mirror layer, which is arranged between the semiconductor layer stack and the base. The n-conducting semiconductor layer faces away from the base, and the n-conducting semiconductor layer or an outcoupling layer located on the n-conducting semiconductor layer has a radiation-outcoupling surface which, in turn, includes a plurality of planar outcoupling sub-surfaces, which are positioned obliquely with respect to a main plane of the radiation-generating region and each form an angle of between 15° and 70° with this plane.

In one embodiment, the outcoupling subsurfaces form pyramid-like structures.

In another embodiment, the mirror layer includes a highly reflective layer, and/or a protective layer, and/or a joining layer.

In still another embodiment, the semiconductor layer stack includes at least one trench, which defines a plurality of individual semiconductor layer elements. The semiconductor layer elements each have a diameter or a width, which includes at most ten pyramid-like structures. The trench(es) are at least sufficiently deep for them to isolate at least the radiation-generating region, and the width of the trench or trenches is at least double the depth of the trenches.

In yet another embodiment, the semiconductor chip is a thin-film component, from which a growth substrate wafer is at least partially removed after the epitaxially produced semiconductor layer stack has been grown.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, advantages and expedient configurations will emerge from the four exemplary embodiments relating to the device and four exemplary embodiments relating to the method which are explained below in conjunction with FIGS. 1 to 8. In the drawings:

FIGS. 1 a and 1 b each show a diagrammatic sectional view of a first exemplary embodiment of a semiconductor chip according to the invention,

FIGS. 2 a and 2 b each show diagrammatic plan views of a surface which is provided with pyramid-like structures in accordance with the invention,

FIG. 3 shows a diagrammatic sectional view of a second exemplary embodiment of a semiconductor chip according to the invention,

FIG. 4 shows a diagrammatic sectional view of a third exemplary embodiment of a semiconductor chip according to the invention,

FIGS. 5 a and 5 b show a diagrammatic sectional view and a diagrammatic plan view, respectively, of a fourth exemplary embodiment of a semiconductor chip according to the invention,

FIGS. 6 a to 6 e diagrammatically depict a first exemplary embodiment of a fabrication method according to the invention,

FIGS. 7 a and 7 b diagrammatically depict a second exemplary embodiment of a fabrication method according to the invention, and

FIGS. 8 a and b diagrammatically depict a third exemplary embodiment of a fabrication method according to the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Elements which are identical or have an identical function are provided with identical reference symbols throughout the figures. In particular, the thickness of the layers is not depicted to scale in the figures, in order to make it easier to understand the invention.

The semiconductor chip illustrated in FIG. 1 a has a semiconductor layer stack 1 which includes an n-conducting semiconductor layer 11, an active region 12 which generates electromagnetic radiation and a p-conducting semiconductor layer 13. All the layers 11, 12 and 13 consist of nitride III-V compound semiconductor material from the system InxAlyGa1-x-yN where 0 (x (1, 0 (y (1 and x+y (1. The active region 12 may have a pn junction, a double heterostructure, a single quantum well structure (SQW structure) or a multiple quantum well structure (MQW structure). Structures of this type are known to the person skilled in the art and are therefore not explained in more detail at this point.

The surface of the p-conducting semiconductor layer 13 is provided with pyramid-like structures 15. These pyramid-like structures 15 are overlaid with a mirror layer 40 over the entire surface.

The mirror layer 40 comprises, for example, three layers, namely a highly reflective layer 41, a protective layer 42 and a joining layer 43. The highly reflective layer 41 includes, for example, silver or aluminium and lies directly on the p-conducting semiconductor layer 13. Since silver and aluminium have a good electrical conductivity, the mirror layer 40 can at the same time serve as a contact layer for the p-conducting semiconductor layer 13. The protective layer 42 contains, for example, titanium nitride and lies on the highly reflective layer 41. The joining layer 43 consists, for example, of gold, tin or an alloy of these metals and is arranged between the protective layer 42 and a base 50 (not shown). The patterned surface of the p-conducting semiconductor layer 13 together with the highly reflective layer 41 forms a reflection surface 131. A radiation outcoupling surface 111 is formed by the surface of the n-conducting semiconductor layer 11.

The semiconductor layer stack 1 includes, for example, GaN, and the p-conducting semiconductor layer 13 may be doped with magnesium. The semiconductor layer stack 1 may also comprise other compounds comprising the elements belonging to the third and/or fifth main groups of the periodic system, for example InGaN, InN, AlInGaN, AlGaN and/or AlN. The n-conducting semiconductor layer 13 is, for example, 2 (m thick, and the p-conducting semiconductor layer 0.5 (m thick. The electrical contacting of the semiconductor layer stack is not illustrated in FIG. 1 a.

Unless stated otherwise, the above materials and dimensions also apply to the other exemplary embodiments relating to the device and the method.

FIG. 1 b shows a beam 3 which is emitted in the direction of the reflection surface 131 and is reflected twice on the reflection surface 131 provided with pyramid-like structures. Specifically, the beam 3 is emitted with an angle ((relative to the vertical) from the radiation-generating region 12 and is firstly reflected on a sub-surface 14 a and then a second time on a sub-surface 14 b which is adjacent to the sub-surface 14 a. After this double reflection, the beam 3 can impinge on the radiation outcoupling surface 111 at an angle ((relative to the vertical) which is smaller than the angle (with which it is emitted from the radiation-generating region 12.

A crucial factor for this behaviour is the angle (which the sub-surfaces of the pyramid-like structures form with a main plane of the radiation-generating region 12. If the angle (is smaller than the critical angle for total reflection (c, the beam 3 is not totally reflected, but rather is decoupled. If the textured reflection surface 131 acts in such a way that the angle (of a beam, after the beam has been reflected twice, is smaller than the initial angle (with respect to the abovementioned main plane, the beam is decoupled. By contrast, the angle (after reflection on a planar mirror is approximately equal to the angle (, i.e. a flat mirror does not substantially change the angle of a beam.

On the other hand, a greater number of reflections is unfavourable because repeated reflection at the reflection surface and/or at the surface from which light emerges, rapidly causes the intensity of the beam to drop on account of absorption in the chip. Nevertheless, a beam which has already been totally reflected at the radiation outcoupling surface 111 can still have the possibility of being decoupled by double reflection on the textured reflection surface 131, provided that the intensity of the beam is sufficient.

FIG. 2 a shows a surface which is provided with a plurality of pyramid-like structures 15. By way of example, each pyramid-like structure 15 comprises six adjacent planar sub-surfaces 14 which are positioned obliquely with respect to a main plane of the radiation-generating region 12. The optimum angle which each sub-surface 14 forms with this main plane depends on whether a reflection surface 131 or a radiation outcoupling surface 111 is concerned.

In the case of a reflection surface 131, each of the sub-surfaces 14 preferably form an angle (of between 10° and 50° with the main plane. This range of angles increases the probability that, after double reflection on the oblique sub-surfaces 14, the angle (will be smaller than the critical angle for total reflection (c. By way of example, the pyramid-like structures 15 each have a base with a width or diameter of 2 (m and a height of 250 nm, corresponding to an angle of approx. 15°.

For a radiation outcoupling surface 111, each of the sub-surfaces 14 preferably form an angle (of between 15° and 70° with the main plane.

FIG. 2 b shows a single pyramid-like structure 15, which consists of six adjacent oblique sub-surfaces 14. Pyramid-like structures which consist of three or more oblique sub-surfaces 14 are also possible. A combination of pyramid-like structures consisting of a different number of sub-surfaces 14, whose sub-surfaces and/or bases are of different sizes, is also possible.

FIG. 3 shows a semiconductor chip which has a radiation outcoupling surface 111, which is provided with pyramid-like structures, on the n-conducting GaN-based semiconductor layer 11. The pyramid-like structures have, for example, a pyramid base with a mean diameter of approx. 3 (m and a height of between 100 nm and 2 (m. In the case of a radiation outcoupling surface 111, the sub-surfaces 14 preferably form an angle (of between 15° and 70° with the said main plane.

The fact that the maximum angle (for the radiation outcoupling surface 111 is larger than for the reflection surface 131 is attributable to the fact that a beam which has already been totally reflected can also be decoupled at an opposite sub-surface 14 of the pyramid-like structure 15. The fact that steeper angles are also optimal is advantageous for the radiation outcoupling surface 111, because steeper angles are easier to produce than small angles when patterning by means of masking etch processes. In this case, by way of example, a planar mirror layer 40 is arranged on the p-conducting semiconductor layer 13. The mirror layer 40 is in turn arranged on a base (not shown).

FIG. 4 shows a further variant, which has a radiation outcoupling surface 111 provided with pyramid-like structures on an outcoupling layer 16, which in turn is arranged on the n-conducting semiconductor layer 11. The outcoupling layer 16 consists, for example, of SiC and may be an unremoved part of the growth substrate wafer 10. A planar mirror layer 40 is arranged on the p-conducting semiconductor layer 13. This results in a planar reflection surface 131 being produced between the p-conducting semiconductor layer 13 and the mirror layer 40.

Similarly for the semiconductor chip illustrated in FIG. 3, the semiconductor chip shown in FIG. 5 a has a plurality of pyramid-like structures on the n-conducting semiconductor layer 11, but additionally also includes at least one trench 17. The trench(es) 17 extend all the way through the n-conducting semiconductor layer 11 and at least as far as the radiation-generating region 12 of the semiconductor stack 1 and are, for example, approx. 2 (m deep and 4 (m wide. The cross section of the trenches is, for example, quadrilateral, although the trench may also take other shapes. The trenches 17 define semiconductor layer elements 18. Each side face, which delimits the trench 17 of the semiconductor stack 1 forms an additional radiation outcoupling surface and thereby increases the outcoupling of light from the semiconductor chip.

The trenches 17 are advantageously sufficiently wide for the radiation to be decoupled directly at the sides without it impinging on an opposite side face of an adjacent semiconductor layer element 18. If the radiation were to impinge on an opposite side face, there would be a risk of it being at least partially absorbed there.

FIG. 5 b illustrates a plan view of a semiconductor chip of this type in which the semiconductor layer elements 18 form, for example, hexagonal shapes on a base 50. An individual semiconductor chip of this type includes a plurality of semiconductor layer elements 18. The size of the semiconductor layer elements 18 preferably depends on the size of the pyramid-like structures 15. The semiconductor layer elements 18 illustrated in FIG. 5 a each have, by way of example, three pyramid-like structures 15 over their mean diameter. The semiconductor layer elements 18 advantageously have a mean diameter which includes fewer than ten pyramid-like structures. As seen from above, the semiconductor layer elements 18 may also be in the shape of circles or may take other shapes.

FIGS. 6 a to 6 e provide an outline illustration of a number of method steps used to fabricate a semiconductor chip illustrated in FIG. 1 a. In FIG. 6 a, a growth substrate wafer 10, which consists, for example, of SiC, is provided and the n-conducting semiconductor layer 11 is deposited epitaxially on the growth substrate wafer 10. Then, the active region 12 and the p-conducting semiconductor layer 13 are grown epitaxially. The deposition conditions (for example the deposition temperature, deposition time, doping level) are selected in such a way that pyramid-like structures whose sub-surfaces form the desired angles with the main plane, namely between 10° and 50°, are formed on the p-conducting semiconductor layer 13.

Then, a mirror layer 40 is applied to the entire surface of the textured reflection surface 131 (FIG. 6 b). Through the pyramid-like structures which are grown on the p-conducting semiconductor layer 13, a reflection surface 131 is formed between the p-conducting semiconductor layer 13 and the mirror layer 40. A base 50 is adhesively bonded or soldered to the surface of the mirror layer 40 which lies on the opposite side from the reflection surface 131. The mirror layer 40 includes, for example, a joining layer, through which the base 50 can be adhesively bonded or soldered to the semiconductor layer stack 1 (FIG. 6 c). The base 50 consists, for example, of gallium arsenide or copper.

The growth substrate wafer 10 is removed from the semiconductor layer stack 1, for example by means of a laser lift-off method, a sacrificial layer, a layer provided with predetermined breaking points or any other method which is known to the person skilled in the art (cf. FIG. 6 d). The n-conducting semiconductor layer 11 is then provided with a contact layer 2 which includes, for example, indium tin oxide (ITO) or ZnO. The p-conducting semiconductor layer 13 is already electrically contacted by means of the electrically conductive mirror layer 40. The chips are then separated, for example, by sawing (cf. FIG. 6 e).

In a variant of the fabrication method illustrated in FIG. 6, the growth substrate wafer 10 illustrated in FIG. 6 c is not completely removed (cf. FIG. 7 a). The remaining part of the growth substrate wafer 10 is then patterned with pyramid-like structures (cf. FIG. 7 b). The patterning can be carried out by means of a chemical process (e.g. an etching process) or a mechanical process (e.g. grinding, sawing or milling). This results in a textured radiation outcoupling surface 111 which consists, for example, of SiC. This textured layer formed from part of the growth substrate wafer 10 corresponds to the layer referred to as the outcoupling layer 16 in FIG. 4. This outcoupling layer 16 is then suitably patterned in order for electrical contact to be made with the n-conducting semiconductor layer 11. On account of the electrically conductive mirror layer 40, electrical contact of the p-conducting semiconductor layer 13 has already been established on the surface. The chips are then separated.

In a further variant of the fabrication method illustrated in FIG. 6, the n-conducting semiconductor layer 11 is textured with pyramid-like structures after the growth substrate wafer 10 has been completely removed. This textured surface serves as the radiation outcoupling surface 111 and can be produced by means of mechanical or chemical processes. The way in which electrical contact of the n-conducting semiconductor layer 11 is established and the chips are separated corresponds to that which has already been described above.

Furthermore, before contact is made with the n-conducting semiconductor layer 11, trenches 17 can be produced in the semiconductor layer stack 1, for example in order to fabricate the semiconductor chip illustrated in FIG. 5 a. These trenches can be produced, for example, by means of an etching process combined with photolithography. First of all, a mask layer is applied to the n-conducting semiconductor layer 11 or the outcoupling layer 16 and is suitably patterned in order to define the desired shape and arrangement of semiconductor layer elements 18. Those parts of the semiconductor layer stack 1 or of the outcoupling layer 16 which are not covered by the mask layer are etched until the trenches 17 formed by etching, in terms of their depth, at least reach the radiation-generating region 12. By way of example, the trenches in the exemplary embodiment illustrated in FIG. 5 a have been etched all the way to the mirror layer 40. After the mask layer has been removed, contact can be made with the chips and the chips can be separated.

The above-described variants of the fabrication method illustrated in FIG. 6 can also take place without the patterning of the p-conducting semiconductor layer 13, so that in such cases a planar mirror layer is provided.

The present patent application claims the priority of German patent application 102 45 628.3-33, the disclosed content of which is hereby incorporated by reference.

The scope of protection of the invention is not limited by the description of the invention on the basis of the exemplary embodiments. Rather, the invention comprises any novel feature and any combination of features, which in particular includes any combination of features which are stated in the patent claims, even if this combination of features is not explicitly stated in the patent claims. 

1. A semiconductor chip which emits electromagnetic radiation, comprising: an epitaxially produced semiconductor layer stack based on nitride semiconductor material, which includes an n-conducting semiconductor layer, a p-conducting semiconductor layer and an electromagnetic radiation generating region which is arranged between these two semiconductor layers; a base, on which the semiconductor layer stack is arranged; and a mirror layer comprising a highly reflective layer, a protective layer and a joining layer, said mirror layer being arranged between the semiconductor layer stack and the base; wherein the n-conducting semiconductor layer faces away from the base; and wherein the n-conducting semiconductor layer or an outcoupling layer located on the n-conducting semiconductor layer has a radiation-outcoupling surface which in turn includes a plurality of planar outcoupling sub-surfaces which are positioned obliquely with respect to a main plane of the radiation-generating region and each form an angle of between 15° and 70° with this plane.
 2. The semiconductor chip which emits electromagnetic radiation according to claim 1, wherein the outcoupling sub-surfaces form pyramid-like structures.
 3. The semiconductor chip which emits electromagnetic radiation according to claim 1, wherein the semiconductor layer stack includes at least one trench which defines a plurality of individual semiconductor layer elements.
 4. The semiconductor chip which emits electromagnetic radiation according to claim 3, wherein the semiconductor layer elements each have a diameter or a width which includes at most ten pyramid-like structures.
 5. The semiconductor chip which emits electromagnetic radiation according to claim 3, wherein the trench(es) are at least sufficiently deep for them to isolate at least the radiation-generating region.
 6. The semiconductor chip which emits electromagnetic radiation according to claim 3, wherein the width of the trench or trenches is at least double the depth of the trenches.
 7. The semiconductor chip which emits electromagnetic radiation according claim 1, wherein the semiconductor chip is a thin-film component, from which a growth substrate wafer is at least partially removed after the epitaxially produced semiconductor layer stack has been grown.
 8. The semiconductor chip which emits electromagnetic radiation according to claim 1, wherein the highly reflective layer includes silver or aluminum.
 9. The semiconductor chip which emits electromagnetic radiation according to claim 1, wherein the protective layer contains titanium nitride.
 10. The semiconductor chip which emits electromagnetic radiation according to claim 1, wherein the joining layer comprises gold, tin or an alloy comprised of gold and tin. 